A radio frequency (RF) linear amplifier, such as a radio transmitter power amplifier, is an amplifier that increases the power of a modulated carrier signal while preserving both the amplitude and phase modulation components of the signal. Whereas, a saturating RF amplifier, commonly used in frequency-modulation (FM) radio transmitters, preserves phase modulation of the signal, but may remove or severely distort any amplitude modulation (AM) component. FM signals typically have constant amplitude, and thus, do not require linear amplification. However, more sophisticated communication systems, such as those receiving and transmitting digital data, may require both amplitude and phase modulation (PM) of the transmitted signals. Linear amplifiers are therefore necessary to transmit those signals.
Negative feedback principal is commonly used to provide substantially linear amplifiers. One such linear RF amplifier generally comprises a feedback loop circuit having an amplifier in the forward path which provides an amplified RF out signal. Typically, the RF output signal is demodulated to provide a feedback signal at baseband. The feedback signal is applied to a summer in the forward signal path to close the loop circuit.
In practice, RF circuits are typically located at some distance from the baseband circuitry. The separation of the RF circuits and the baseband circuits introduces a time delay which appears as additional phase shift in the feedback signal causing the overall phase margin of the loop circuit to be reduced. The reduction of phase margin may cause oscillation at certain transmit frequencies.
One approach in compensating the phase shift is to measure a phase error within the feedback loop. A known method for measuring the phase shift requires an initial setup procedure. A disadvantage of this approach, however, is that the feedback loop must be broken, i.e. opened, to measure the phase error of the loop. Additionally, the phase error correction circuitry must be preset to provide optimum results. Therefore, there is a need to provide a simplified approach for compensating the phase shifts caused by the path time delays in the linear amplifier circuit.